IRLF 


SB 


bfl? 


GIFT   OF 

i/ir.H.L.Leupp 


FOR  OFFICIAL  USE  ONLY 


ELEMENTARY 

ELECTRICITY 


Training  Pamphlet 
No.  1 


Signal  Corps,  U.  S.  Army 

5-10-18 


Elementary  Electricity 


Electricity  and  magnetism  are  the  two  things  which  form 
the  basis  of  most  signalling  work  in  modern  warfare.  What 
they  are  is  impossible  to  state,  but  there  need  not  be  so  much 
concern  with  that  as  with  what  they  do. 

Magnetism  is  a  property  possessed  by  some  metals  whereby 
they  attract  or  repel  other  pieces  of  metal.  If  small  pieces 
of  iron  or  steel  are  placed  near  a  permanent  steel  magnet, 
they  will  be  attracted,  and  when  the  force  is  great  enough 
will  jump  to  the  magnet.  This  is  ^hat  happens  wh#n  :Che 
armature  of  a  telegraph  instrument  $umo&  qve.f  againsc  -the 
pole  of  the  magnet  to  produce  the  .familiar  cjick,, 

If,  instead  of  testing  this  magnetism,  with-  a  steferonagnet 
and  ordinary  pieces  of  iron,  two  steel  magnets  are  used,  it  is 
discovered  at  once  that  in  one  position  the  ends  of  the  bars 
strongly  attract  each  other,  while  if  one  of  the  bars  is  re- 
versed, they  repel  each  other,  Fig.  1.  This  indicates  that 
there  is  a  difference  between  the  magnetism  at  the  two  ends 
of  a  bar.  Since  the  earth  is  magnetic  toward  the  poles,  it  has 
been  agreed  that  the  end  of  the  magnet  which  is  attracted 
toward  the  north  shall  be  calle  i  the  "north  pole"  of  that 
magnet,  and  that  the  opposite  end  shall  be  called  the  "south 
pole." 

The  law  of  magnetism  is  that  unlike  poles  attract  and  like 
poles  repel. 

It  should  be  noted  that  inasmuch  as  the  north  pole  of  the 
magnet  is  attracted  toward  the  north  pole  of  the  earth,  it  is  in 
reality  a  south  magnetic  pole.  The  magnetic  pole  of  the 
earth  is  not  exactly  at  the  geographic  pole.  There  are  also 
magnetic  variations  due  to  the  makeup  of  the  earth  which 
change  with  time.  Consequently,  a  compass  made  of  a  small 
pivoted  permanent  magnet  does  not  point  directly  north, 
Fig.  2,  but  has  a  slight  deviation  from  this  true  north  direc- 
tion which  is  called  the  "magnetic  deviation."  To  get  the  true 
north  from  a  compass,  this  deviation,  which  varies  for  dif- 
ferent points  on  the  earth's  surface,  must  be  known. 

Methods  of  Magnetizing 

A  bar  of  steel  may  be  magnetized  by  bringing  it  near  to 


667306 


another  strong  magnet,  Fig.  3,  by  stroking  it  with  another 
magnet,  Fig.  4,  or  by  bringing  it  under  the  magnetic  in- 
fluence that  always  surrounds  an  electric  current,  Fig.  5. 
Steel  will  retain  most  of  this  magnetism,  and  it  is  therefore 
called  a  "permanent  magnet."  A  piece  of  soft  iron  becomes  a 
magnet  under  the  same  influence,  in  fact,  a  much  stronger 
magnet  than  a  similar  piece  of  steel,  but  when  the  inducing 
force  is  removed,  all  but  a  small  residual  magnetism  is  lost. 

Theory  of  Magnetism 

The  supposition  that  seems  to  best  explain  the  observed 
of  .magnetism,  is  that  the  metal  is  made  up  of 
1  molecules,  each  of  which  is  a  small 
^with.Bprth  and  south  poles.  As  these  are 
^  p,^  unlike  poles  will  tend  to  point 

towards  each  other*  insitfe  the  metal  and  there  will  be  no 
total  outside  magnetic  effect  from  the  whole  piece.  Under 
the  influence  of  magnetism,  the  molecules  become  oriented  so 
that  they  all  point  in  one  direction  magnetically,  and  their 
total  effect  is  added.  By  cutting  up  a  long  permanent  mag- 
net, it  is  found  that  each  piece  becomes  a  complete  magnet, 
this  supporting  the  molecular  theory.  It  is  easier  for  the 
molecules  to  turn  in  a  piece  of  soft  iron  than  it  is  in  steel, 
so  that  the  effect  is  greater  in  iron.  But  in  steel,  once  they 
are  turned,  they  are  more  likely  to  remain  that  way.  It 
should  be  remembered  that  permanent  magnets  must  not  be 
struck  a  sharp  blow  for  this  disturbs  the  magnetic  balance 
and  tends  to  assist  the  molecules  to  return  to  their  former 
positions  with  a  consequent  loss  of  magnetism  in  the  total 
piece.  The  positions  of  the  molecules  in  a  bar  before  and 
after  the  influence  of  magnetism  are  shown  in  Fig.  6. 

The  effect  of  magnetism  passes  through  any  substance.  It 
cannot  be  insulated.  However,  it  can  all  be  made  to  pass 
within  an  iron  or  steel  casing,  as  in  this  case,  the  surround- 
ing iron  forms  a  good  conductor  for  the  magnetic  lines  of 
force,  which  then  tend  to  follow  the  path  of  least  resistance?. 
The  lines  of  force  can  thus  be  controlled  in  a  large  measure. 

A  medium,  called  "ether,"  is  assumed  to  be  present  in  all 
matter  and  space.  Magnetism  is  considered  to  be  strains  in 
this  ether.  These  strains  grade  off  in  intensity  outside  the 
magnet  from  one  pole  to  the  other.  Iron  filings  sprinkled  on 


a  paper  over  a  permanent  magnet  take  up  positions  in  curved 
lines  extending  from  one  pole  to  the  other,  similar  to  that 
shown  in  Fig.  3.  These  lines  represent  the  directions  along 
which  the  ether  strains  change  uniformly  and  are  called  the 
"magnetic  lines  of  force." 

Electricity 

Electricity  is  assumed  to  consist  of  small  particles  called 
electrons.  When  these  follow  each  other  along  a  course,  as 
they  do  most  easily  in  a  wire,  the  result  is  an  electric  cur- 
rent which  is  the  all  important  thing  in  signalling.  In  order 
to  have  a  flow  of  electric  current,  there  must  be  more  elec- 
trons in  one  portion  of  the  circuit  than  in  another.  Such  a 
condition  may  be  brought  about  by  the  use  of  a  battery  or  of 
a  machine  which  generates  electricity,  called  a  dynamo  or 
generator.  There  is  an  average  concentration  of  these  elec- 
trons all  over  the  surface  of  the  earth,  which  is  considered 
to  be  the  zero  potential.  These  electrons  have  been  found  to 
have  a  negative  charge.  From  the  assumption  scientists  orig- 
inally made,  a  body  having  more  than  the  average  earth  con- 
centration of  these  electrons  is  considered  to  have  a  neg- 
ative electric  potential.  If  it  has  less  than  the  average 
concentration  of  electrons,  it  has  a  positive  electric  poten- 
tial. The  original  assumption  of  the  direction  of  cur- 
rent flow  was  made  by  scientists  previous  to  the  discovery 
of  the  electron  theory  for  explaining  the  flow  of  current,  an£ 
since  then  their  assumption  has  been  taken  quite  generally 
to  be  wrong.  By  this  is  meant,  the  direction  of  movement  of 
the  electrons  is  from  negative  to  positive,  while  the  direction 
of  the  electric  current  was  assumed  to  be  from  positive  to 
negative,  Fig.  7.  However,  in  elementary  electricity,  there  is 
not  so  much  concern  with  electrons  as  there  is  with  the  di- 
rection of  flow  and  quantity  of  current. 

Bodies  electrically  charged  behave  in  some  ways  like  the 
poles  of  magnets,  that  is,  like  charges  repel  and  unlike 
charges  attract.  If  two  objects  have  unequal  concentration  of 
electrons,  and  they  are  connected  together,  the  electrons  will 
flow  along  the  wire  to  balance  up  the  concentration.  The 
direction  of  this  flow  will  be  from  the  object  having  the 
greater  concentration  of  electrons,  or  the  negative  electric 
potential,  to  the  object  having  the  lesser  concentration  of 


electrons,  or  the  positive  electric  potential.  The  direction  of 
current  flow,  according  to  the  old  assumption,  is  then  just  the 
opposite,  from  positive  to  negative. 

If  there  is  a  steady  flow  of  current  in  one  direction,  this 
is  called  a  "direct  current,"  Fig.  8.  If  the  current  flows  first  in 
one  direction  and  then  in  the  opposite  direction,  this  is  called 
an  "alternating  current,"  Fig.  9.  If  it  always  flows  in  one 
direction,  but  its  value  changes  between  the  limits  of  a  maxi- 
mum and  zero,  it  is  then  called  a  "pulsating  current." 

Primary  Batteries 

As  stated  above,  in  order  to  secure  a  flow  of  current,  it  is 
necessary  to  have  a  difference  of  potential  existing  in  the 
circuit.  One  of  the  simplest  ways  of  bringing  this  about  is 
by  means  of  a  chemical  cell.  Such  a  cell  transforms  chemical 
energy  into  electrical  energy,  and  is  known  as  a  "primary 
cell."  There  are  a  large  number  of  possible  combinations  of 
metals  and  chemicals  which  can  be  used  to  make  up  an  or- 
dinary primary  cell,  some  being  more  effective  than  others, 
and  each  having  its  respective  advantages.  In  simplest  form, 
the  primary  cell  consists  of  two  different  metal  plates  sus- 
pended in  an  acid  solution,  called  the  "electrolyte."  This  acid 
has  different  effects  upon  the  two  plates,  Fig.  10.  For  ex- 
ample, with  copper  and  zinc  as  the  two  metals  suspended  in 
a  dilute  solution  of  sulphuric  acid,  the  zinc  is  much  more 
readily  attacked  by  the  acid  than  the  copper.  If  these  plates 
are  connected  together  by  means  of  a  wire  outside  of  the 
cell,  a  vigorous  chemical  action  takes  place,  the  zinc  dis- 
solving in  the  acid  to  form  zinc  sulphate  and  liberate  hydro- 
gen. The  hydrogen  liberated  carries  a  negative  charge  of 
electricity  which  travels  across  the  acid  to  the  copper  plate, 
there  giving  up  its  charge  to  the  copper  plate  and  passing 
off  as  gas.  This  causes  a  difference  of  potential  on  the  two 
plates  so  that  a  current  flows  in  the  exterior  circuit 

This  difference  of  potential  may  also  be  explained  by  re- 
verting to  electrons.  The  reaction  of  the  acid  with  the  metal 
changes  the  physical  shape  very  slightly,  of  course,  in  a  unit 
of  time,  and  in  consequence  of  this  eating  away  of  the  metal 
brings  about  a  change  in  the  density  of  the  electrons  'on  the 
metal.  As  this  rate  of  change  on  the  two  metals  is  unequal, 
there  is  consequently  a  difference  of  potential  s.et  up  between 


the  two  plates.  The  electric  potential  or  voltage  of  a  cell 
therefore  does  not  depend  upon  the  size  of  the  cell  but  only 
on  the  elements  used.  However,  the  quantity  of  electricity 
which  can  be  easily  taken  from  a  cell,  and  the  speed  with 
which  it  can  be  drawn  off,  do  depend  on  the  magnitude  of  the 
reaction  taking  place;  that  is,  the  larger  the  cell  the  greater 
the  current  supply  available,  Fig.  10. 

The  metals  which  may  be  used  as  the  poles  of  a  cell  are 
listed  below  according  to  their  relative  potential  when  dipped 
into  a  solution  acting  as  an  electrolyte.  Those  near  the  top 
of  the  list  are  positive  to  those  below.  This  list  is  known  as 
the  electro-chemical  series. 
Positive  Terminal. — Carbon. 

Platinum. 

Silver. 

Copper. 

Lead. 

Tin. 

Iron. 

Zinc — Negative  Terminal. 

The  maximum  electric  pressure  or  voltage  that  can  be  ob- 
tained from  any  combination  of  these  metals,  is  about  three 
volts,  whereas  the  current  capacity  is  limited  only  by  the  size 
of  the  cell. 

The  ordinary  dry  cell  consists  of  a  zinc  cylinder  which 
serves  as  a  container  and  acts  as  one  plate  or  pole  of  the 
cell.  This  container  is  filled  with  a  moist  mixture  which  acts 
as  the  electrolyte.  A  central  carbon  cylinder  or  plate  forms 
the  positive  pole.  As  cells  are  used,  gases  collect  at  the  plates 
which  tend  to  reduce  the  chemical  action  and  lower  the  volt- 
age and  output  of  the  cell.  The  cell  is  then  said  to  be  "polar- 
ized." This  is  lessened  to  a  certain  extent  by  the  help  of  a 
chemical  substance  which  acts  to  absorb  this  gas  and  is 
called  a  "depolarizing  agent."  But  in  spite  of  the  depolarizer, 
a  battery  gradually  becomes  polarized  and  its  usefulness  dis- 
sipated. Such  a  battery  may  be  renewed  and  further  output 
obtained,  in  case  of  emergency,  by  heating  it. 

Storage  Batteries 

The  secondary  or  storage  cell,  which  also  finds  extensive 
use  in  Signal  Corps  work,  is  one  which  generates  electricity 


Attract 


FIO.  9 


FI0.3 


Magnetic  Lines  of  Force 


1.5  volts 
15  amperes    1  ampere 


in  Series 
rolts 


y 


10 


M; 


12   12  fe  12  12  volts 
Batteries  in  Parallel 


FIO.  6  Eloctricm 

Magnetization 


FIG.  12 


I^loPan^re" 


no.  c 


Iron  Bar,Deoagnetized, 

ES 


20 


50  amperes 


"rHWf 

aya-       t  «•  +  f   T- 


FIO.  7          Electric  Current  now 

phy^oooaaooobooo  [T| 


Electron  Flow 
PIG.  8 


Multiple  Series 
Connection 

10  volts 
FIG.  13 


Direct  Current 


by  means  of  chemical  reaction,  just  as  in  the  primary  cell. 
It  differs  from  the  primary  coll,  however,  in  that  when  the 
chemical  action  has  gone  on  to  such  a  point  that  the  battery 
is  exhausted,  it  may  be  restored  to  its  former  or  charged  con- 
dition by  connecting  it  to  some  source  of  electricity  for  a 
period  of  time.  In  such  a  cell,  the  chemical  reactions  are 
reversible;  that  is,  the  two  plates  or  poles  of  the  cell  are 
made  up  of  such  materials  as  may  be  chemically  changed  due 
to  the  passage  of  an  electric  current.  And  then  upon  using 
the  cell  as  a  source  of  electricity,  this  chemical  reaction  re- 
verses itself  to  generate  the  electricity  which  has  been  stored 
through  the  previous  chemical  action. 

The  ordinary  primary  cell  will  respond  to  such  a  treatment 
only  to  a  very  slight  degree.  The  elements  which  perform 
this  reversible  function  most  satisfactorily,  are  either  lead 
plates  coated  with  an  active  material  of  lead  oxide  and  lead 
peroxide  inserted  in  a  dilute  solution  of  sulphuric  acid,  or 
steel  plates  containing  nickel  and  nickel  hydrate  and  iron 
oxide  inserted  in  an  electrolyte  of  potassium  hydroxide.  The 
details  of  the  chemical  reactions  taking  place  in  both  of  these 
types  of  storage  batteries,  and  full  information  as  to  care  and 
charging,  etc.,  are  given  in  Radio  Pamphlets  No.  8  and  No. 
8-A,  which  are  available  when  requested  upon  proper  author- 
ity from  the  Land  Division,  Office  of  the  Chief  Signal  Officer 
of  the  Army. 

In  both  primary  and  secondary  cell  considerations,  the 
"cell"  is  the  individual  generating  element.  Any  group  of 
these  cells  for  producing  a  given  power  supply  is  called  a 
"battery." 

Battery  Connections 

The  voltage  of  a  single  cell,  either  primary  or  secondary,  is 
generally  insufficient  to  do  the  work  required.  Batteries  are 
therefore  connected  up  in  combinations  to  deliver  the  output 
and  voltage  desired.  Cells  are  connected  in  series,  Fig.  11, 
when  it  is  desired  to  build  up  the  voltage.  This  is  done  by 
connecting  the  positive  terminal  of  one  cell  to  the  negative 
terminal  of  the  next.  If  it  is  desired  to  obtain  greater  cur- 
rent through  the  exterior  circuit,  larger  cells  must  be  used, 
or  what  amounts  to  the  same  thing,  the  like  plates  of  several 
cells  must  be  connected  together  so  that  they  all  act  together. 


8 

This  gives  the  same  voltage  as  one  cell  but  increases  the 
quantity  of  electricity  available.  This  is  called  "multiple"  or 
"parallel"  connection,  Fig.  12.  When  it  is  desired  to  increase 
both  voltage  and  current,  it  is  necessary  to  make  a  connec- 
tion which  is  a  combination  of  the  series  and  multiple  meth- 
ods mentioned  above,  and  called  the  "series-multiple"  and 
"multiple-series"  connections.  Any  combination  of  cells  in  this 
method  of  connection  may  be  made;  for  instance,  the  cells 
may  be  connected  as  shown  in  Fig.  13  so  that  there  will  be 
two  groups  of  five  cells  each  in  series,  with  the  groups  con- 
nected in  parallel;  or  as  shown  in  Fig.  14  so  that  there  may 
be  two  groups  of  five  cells  each  in  parallel,  with  the  groups 
connected  in  series,  or  any  combination  between  these  two 
limits. 

Poorly  made  battery  connections  is  one  of  the  most  common 
sources  of  trouble  in  the  use  of  electrical  instruments.  At 
the  same  time,  it  is  one  of  the  most  easy  to  avoid  and  to 
correct.  In  making  connections,  pincers  should  always  be 
used  to  tighten  down  the  nuts,  and  the  surfaces  should  always 
be  clean  so  that  there  will  be  a  sure  and  firm  contact.  It  is 
important  that  the  pasteboard  covers  on  dry  cells  should  not 
be  broken  or  removed,  since  if  two  of  the  zinc  containers 
touch  each  other,  a  short  circuit  will  result. 

The  direction  of  flow  of  an  electric  current  may  be  de- 
termined by  placing  two  terminals  in  a  salt  solution.  The 
terminal  at  which  the  larger  amount  of  gas  is  given  off  is  the 
negative  terminal,  or  where  the  current  is  flowing  toward  the 
cell.  The  values  of  current  and  voltage  which  are  placed  on 
the  several  illustrations  showing  different  methods  of  bat- 
tery connection,  are  those  which  obtain  when  the  batteries 
used  are  lead  plate  storage  batteries.  If  Edison  storage  bat- 
teries are  used  instead,  the  cell  voltage  averages  1.2  volts, 
and  for  dry  batteries,  it  averages  1.5  volts.  The  output,  of 
course,  depends  upon  the  construction  of  the  cell. 

Electrical  Units 

All  substances  will  conduct  electricity  but  there  is  a  vast 
difference  in  the  ease  with  which  the  electrons  can  pass 
through  different  substances.  The  opposition  which  the 
substance  offers  to  the  passage  of  the  electrons,  or  electric 
current,  is  called  its  "resistance"  and  the  unit  of  resistance 


9 

is  the  "ohm."  The  "international  ohm"  is  the  resistance  of- 
fered to  the  flow  of  an  unvarying  current  by  a  column  of 
mercury  106.3  centimeters  high  and  weighing  14.4521  grams 
at  a  temperature  of  0  deg.  Centigrade.  The  converse  'of 
resistance,  or  the  ease  with  which  a  substance  conducts  elec- 
tricity, is  called  its  "conductivity."  A  piece  of  copper  wire 
1/10  inch  in  diameter  (No.  10  B.  &  S.  gage)  1,000  feet  long, 
has  a  resistance  of  1  ohm.  The  resistances  of  various  metalss 
and  other  substances  relative  to  the  resistance  of  copper, 
which'  is  taken  as  the  standard,  are  given  in  the  following 
table.  It  should  be  remembered  that  the  lower  the  resistance 
the  better  the  conductor. 


Fair     Conductors. 
Charcoal  and  Coke 
Carbon 
Plumbago 
Acid   Solutions 
Sea  Water 
Salt    Solutions 
Metallic   Ores 
Living    Vegetable 
Moist    Earth 


Partial 

Conductors. 
Water 
The   Body 
Flame 
.Linen 
Cotton 
Dry  Woods 
Marble 


Good    Conductors. 

Silver    '_ .925 

Copper    (annealed)     .975 
Copper    (standard)   1.00 

Gold    1.38 

Aluminum    1.61 

Zinc     ._  3.62 

Platinum     5.65 

Iron     5.70 

Nickel     5.78 

Tin    8.28 

Lead   12.8 

Antimony    _; 22.8 

Mercury     59.3 

Bismuth    82.2 

Very  Poor    Conductors    or    Insulators. 

Slate  Gutta   Percha 

Oils  Shellac 

Porcelain  Ebonite 

Dry  Leather,  Paper  Mica 

Wool  Jet 

Silk  Amber 

Sealing  Wax  Paraffine  Wax 

Sulphur  Glass   (varies  with  quality) 

Resin  Dry   Air 

To  cause  electricity  to  do  any  particular  work,  it  is  neces- 
sary to  get  a  certain  amount  of  it  at  the  spot  where  the  work 
is  to  be  done.  Electricity  is  not  like  water  which  can  be 
stored  in  tanks  at  the  spot  where  it  is  wanted.  The  effect  of 
electricity  comes  from  the  rate  at  which  it  flows  at  the  spot 
in  question.  This  corresponds  to  gallons  of  water  per  minute. 
The  electrical  unit  for  rate  of  flow  is  the  "ampere."  The  "in- 
ternational ampere"  is  that  unvarying  current  which,  when 
passed  through  a  neutral  solution  of  silver  nitrate,  will  de- 
posit silver  at  the  rate  of  .001118  gram  per  second.  (A  con- 
venient way  of  remembering  this  figure  is  that  it  is  made  up 


10 

of  one  point,  two  naughts,  three  ones,  and  four  twos  —  8.)  To 
secure  the  desired  rate  of  flow,  electricity  must  be  forced 
through  the  resistance  between  the  point  of  supply  and  the 
point  of  use,  and  it  must  then  also  be  forced  through  the  ap- 
paratus. A  pressure  must  therefore  exist  behind  the  elec- 
tricity. This  corresponds  to  the  pressure  shown  by  a  steam 
gage  and  in  electrical  quantities  is  called  the  "voltage"  or 
"electromotive  force"  (emf.).  The  unit  of  voltage  is  the 
"volt."  For  all  practical  purposes,  the  volt  may  be  defined  as 
that  emf.  which  will  cause  one  international  ampere  to  fl'ow 
through  one  international  ohm. 

These  three  units  of  resistance,  current  and  voltage  were  so 
chosen  that  the  pressure  of  one  volt  would  force  a  current  of 
one  ampere  through  a  resistance  of  one  ohm.  This  relation 
continues,  so  that  it  requires  twice  as  much  voltage  to  force 
twice  as  much  current  through  the  same  resistance,  or  that 
it  takes  twice  as  much  voltage  to  force  the  same  amount  of 
current  through  twice  the  resistance.  This  relation  is  known 
as  Ohm's  law  and  may  be  expressed  by  the  simple  equation 

E 

I  =  —  , 
R 

where  E  is  the  voltage,  I  the  current  in  amperes,  and  R  the 
resistance  in  ohms.  Knowing  two  of  the  quantities,  it  is 
always  possible  to  find  the  third.  The  law  is  often  expressed 
by  inversions  of  the  equation,  as 

E 
E  =  IR,orR  =  —  . 

I 

The  total  resistance  of  separate  resistances  connected  in 
series,  is  equal  to  the  sum  of  the  individual  resistances. 
Hence  in  Fig  15, 


The  total  resistance  of  a  circuit  which  has  individual  re- 
sistances connected  in  parallel  is  the  sum  of  the  conductivities 
of  the  various  parts,  or  the  sum  of  the  reciprocals  of  the  re- 
sistances. The  greater  the  conductivity  the  less  the  resist- 
ance. Thus  in  Fig.  1C  the  total  resistance  may  be  found 
from  the  simple  equation, 


11 
1111 

R        r         ro        rg 

With  this  knowledge  of  the  resistances  in  a  circuit,  the 
need  for  connecting  batteries  in  different  combinations,  as 
explained  above,  is  more  readily  understood.  For  instance, 
suppose  a  specific  apparatus  requires  a  certain  quantity  of 
electricity  to  make  it  operate,  say  5  amp.  The  total  resist- 
ance of  the  apparatus  and  of  the  wires  leading  to  it,  is  10 
ohms.  In  order  to  supply  the  5  amp.  then,  it  will  be  neces- 
sary to  supply  a  voltage  of  50  volts,  since  by  Ohm's  law, 
E  =  RI,  hence  E  =  10  x  5  =  50  volts.  If  the  resistance  of 
the  exterior  circuit  connected  to  a  cell  is  comparatively  low, 
it  is  necessary  to  take  into  consideration  the  internal  resist- 
ance of  the  battery  in  considering  the  constants  of  the  circuit. 
The  chemical  action  of  the  cells  gives  a  certain  amount  of 
voltage.  This  voltage  must  force  the  current  not  only  through 
the  outside  circuit  but  through  the  cell  itself,  and  this  latter 
becomes  quite  a  large  factor  in  the  flow  of  current  if  the  re- 
sistance of  the  exterior  circuit  is  fairly  low  and  a  large  cur- 
rent is  desired.  If  the  resistance  of  the  exterior  circuit  is 
very  high,  the  cell  resistance  may  be  neglected  without  ma- 
terial error  in  the  computations. 

Suppose  that  the  battery  in  use  in  a  circuit  is  made  up  of 
two  small  cells  connected  in  series.  Each  cell  gives  a  voltage 
of  1.5  volts  normally  and  has  an  internal  resistance  of  .3 
ohm.  The  two  cells  in  series  give  a  battery  voltage  of  3 
volts,  and  resistance  of  .6  ohm.  Suppose,  also,  that  the  ex- 
ternal circuit  has  a  resistance  of  .2  ohm  and  that  it  is  desired 
to  force  6  amp.  through  it.  If  one  of  the  above  batteries  is 
connected  to  the  circuit,  3  volts  are  available  for  forcing  the 
current  through  a  total  resistance  of  .2  _|_  .6  =  .8  ohm.  From 

E        3 
this,  I  =  —  =  —  =  3.75  amperes.     This  is  evidently  not  suf- 

R       .8 

ficient.  Now  place  two  of  these  batteries  in  series,  Fig.  17. 
Tl  is  will  deliver  6  volts  but  will  also  double  the  internal  re- 
sistance, making  it  1.2  ohms,  or  a  total  of  1.4  ohms,  including 

6 

the  external  resistance.  From  this,  I  =  —  =  4.28  amperes.  This 

1.4 


no. 

ItB 


50 


-i 


2  volts 

h 

2 


•HI 


Series  Multiple 
Connection 

—  4  Toltt    - 


FIO.   16 


ri        rri* 

T    Series  Circuit 

I vWA/WWW 1 

'1 


FXO.  16 
Parallel  Circuit 


.20) 


FIO.  17  * 1.2  u 


Effect  of  Electric 
Current  on  Magnets 


FIG.  20 


k 3 


-nets   t 

e 


Compass     Conpass 
Underneath   Above 
the  Wire    the  Wire 


FIG.  21 


.2w 


Yoke 


Wire  moved 
this  direction 


FIG.  23 


Induced  Current 


FIG.  18 


Magnetic  Field  of 
Electric  Current 


FIO.  24 


13 

is  still  not  the  amount  desired,  and  it  is  seen  that  while  the 
source  of  emf.  has  been  doubled,  the  current  which  this  will 
deliver  through  the  exterior  circuit  has  been  increased  only 
a  small  fraction — .5  amp. 

If  the  two  batteries  are  placed  in  parallel,  Pig.  18,  the  volt- 
age of  only  one  will  be  available,  3  volts,  but  the  internal 
resistance  of  the  batteries  will  be  decreased  since  two  paths 
in  parallel  are  provided  for  the  current  to  pass  through.  This 
would  give  a  total  internal  battery  resistance  of  .3  ohm,  ac- 
cording to  the  scheme  shown  above  for  finding  a  total  resist- 
ance of  several  resistances  in  parallel,  and  this  gives  a  total 
resistance  in  the  circuit  of  .3  _[_  .2  =  .5  ohm.  From  this, 
E  3 

I  rr  —  zr  —  =6  amp. 

R        .5 

A  general  rule  which  may  be  followed  in  such  cases  is  to 
connect  the  batteries  in  series  for  use  in  circuits  of  high 
external  resistance.  With  low  external  resistance,  connect  the 
batteries  in  parallel. 

Ohm's  law  may  be  applied  to  any  part  'of  a  circuit.  That 
is,  the  voltage  between  any  two  points  of  a  circuit  is  equal 
to  the  current  flowing  between  those  two  points,  multiplied 
by  the  resistance  between  those  two  points.  Applying  this 
formula  to  various  portions  of  a  circuit,  shows  where  the 
maximum  drop  in  potential  occurs.  For  example,  in  an  elec- 
tric light  circuit,  the  resistance  of  the  light  is  far  greater 
than  that  of  the  line,  so  that  if  115  volts  is  supplied  by  the 
dynamo,  practically  all  of  thr.t  potential  will  be  available  at 
the  lamp  socket.  The  resistance  of  the  wire  to  the  lamp  is  very 
low  in  comparison  to  that  of  the  lamp  itself.  When  a  large 
voltage  is  required  for  forcing  a  certain  current  through  it, 
the  power  must  be  correspondingly  large.  The  unit  of  power, 
in  direct  current  circuits,  is  taken  as  the  product  of  the  volt- 
age and  amperage  and  is  called  the  "watt."  One  watt  of 
power  is  being  applied  when  one  ampere  is  flowing  at  one 
volt  applied  emf.  This  rule  may  be  simply  expressed  by  the 
equation 

W  =  BI. 

For  example,  a  6-amp.  current  flowing  as  the  result  of  a 
3-volt  potential  means  a  power  input  of  18  watts.  In  a  lamp 
circuit,  the  current  is  V2  amp.  at  110  volts,  this  representing 


11 

55  watts  of  power.  A  storage  battery  on  charge  at  a  pressure 
of  4  volts  and  a  current  rate  of  15  amperes  is  taking  60  watts. 

As  a  watt  is  a  rather  small  unit,  1000  watts  is  the  usual 
unit  used  and  this  is  called  a  "kilowatt"  (kw.).  The  elec- 
trical equivalent  of  the  horse-power  unit  is  746  watts.  One 
kilowatt  is  therefore  almost  exactly  1  1/3  hp. 

The  longer  a  certain  number  of  watts  of  power  is  drawn 
from  a  source  of  supply,  the  greater  the  "energy"  which  is 
consumed.  The  unit  of  time  used  in  this  connection  is  the 
hour.  A  watt  of  power  drawn  for  one  hour  is  called  a  "watt- 
hour."  It  is  not  difficult  to  secure  rather  large  currents  for 
a  very  short  time,  but  a  substantial  source  of  supply  must 
be  had  to  maintain  a  considerable  current  for  a  long  time. 
The  watt  hour  is  the  unit  which  expresses  this  requirement. 
As  the  watt-hour  is  a  small  unit,  the  usual  unit  used  com- 
mercially is  the  kilowatt-hour  (kw-hr.),  1000  watt-hours. 

Electromagnetism 

Whenever  there  is  a  flow  of  electric  current,  magnetic 
lines  of  force  are  found  to  circulate  about  it.  The  direction 
of  these  linee  of  force  with  respect  to  the  direction  of  the 
current  can  easily  be  remembered  by  comparison  with  the 
"right  hand  rule."  Consider  the  thumb  to  be  the  electrical 
conductor  in  which  the  current  is  flowing.  Place  the  thumb 
in  such  a  position  that  it  will  point  in  the  direction  the  cur- 
rent is  flowing  in  the  wire.  The  direction  in  which  the  fing- 
ers curve  around  will  then  indicate  the  direction  of  the  mag- 
netic lines  of  force  which  are  present  about  the  wire.  The 
lines  of  force  are  closed  circles,  however,  and  do  not  advance 
along  the  wire  with  the  current,  Fig.  19. 

A  magnetic  compass  placed  over  a  wire  carrying  a  current 
will  be  deflected  to  a  position  at  right  angles  to  the  wire  and 
pointing  in  one  direction,  and  when  placed  underneath  the 
wire,  will  be  deflected  in  the  opposite  direction,  Fig.  20.  This 
serves  as  a  further  indication  of  the  condition  described  by 
the  right  hand  rule.  The  north  pole  of  the  compass  will 
point  in  the  direction  of  the  lines  of  force.  As  the  compass  is 
moved  around  the  wire,  the  needle  will  change  its  position, 
indicating  the  direction  of  the  lines  of  force  all  the  way 
around  the  wire. 

The  magnetism  resulting  from  a  single  straight  wire  is  of 


little  value  because  it  is  so  weak.  But  if  the  similar  effect  of 
a  large  number  of  wires  is  concentrated  in  a  small  space,  the 
magnetic  effect  may  be  of  considerable  moment.  This  may  be 
accomplished  by  coiling  the  wire  so  that  many  turns  pass  around 
the  same  space.  This  concentrates  all  the  lines  of  force  and 
when  current  is  passing  through  the  coil  there  exists  what  is 
commonly  known  as  the  "electromagnet."  The  effect  may  be 
made  even  more  pronounced  by  placing  a  core  of  iron  within 
the  coil,  since  iron  conducts  magnetism  about  1000  times  more 
readily  than  air.  By  this  construction,  the  lines  of  force  created 
by  each  individual  turn  of  wire,  add  up  through  the  iron  core  of 
the  coil  to  produce  very  strong  magnetic  poles  at  the  ends  'of  the 
core.  A  north  magnetic  pole  is  formed  at  one  end  of  the  core 
and  a  south  pole  at  the  opposite  end.  To  determine  at  which 
end  the  north  pole  exists,  the  right  hand  rule  may  again  be 
used.  Place  the  thumb  so  that  it  will  indicate  the  direction 
of  current  flow  in  any  individual  turn  of  wire  of  the  coil, 
and  at  any  position  around  the  turn.  The  position  of  the 
fingers  will  then  indicate  the  direction  of  the  magnetic  lines 
of  force  in  the  core,  or  will  point  towards  the  north  pole  of 
the  magnet,  Fig.  21. 

For  most  purposes,  it  is  better  to  construct  the  magnet  so 
that  the  forces  of  both  the  north  and  south  poles  may  be 
utilized  to  act  on  a  small  piece  of  iron  called  an  armature, 
Fig.  22.  This  is  done  by  placing  an  iron  yoke  across  one  end 
of  two  coils  to  form  a  closed  magnetic  path  from  the  south 
pole  of  one  coil  around  through  the  yoke  to  the  north  pole  of 
the  opposite  coil.  A  bar  of  soft  iron  placed  near  the  open 
end  of  the  yoke  is  strongly  affected  by  both  poles,  tem- 
porarily megnetizing  the  soft  iron  armature  as  indicated. 
Such  an  arrangement  is  essentially  the  construction  used  in 
the  telegraph  sounder,  the  ordinary  door  bell  and  many  other 
electromagnetic  devices. 

Upon  closing  the  circuit  shown  in  Fig.  22,  the  armature 
would  be  drawn  over  against  the  poles  and  held  there  until 
the  circuit  was  again  broken.  In  a  door  bell,  then,  this  would 
give  simply  one  stroke  of  the  bell.  To  make  the  bell  ring 
continuously,  it  is  necessary  to  insert  a  device  for  breaking 
the  circuit  each  time  the  clapper  is  thrown  against  the  bell. 
This  is  simply  done  by  a  spring  contact  on  the  armature 
through  which  the  current  energizing  the  coils  passes.  When 


16 

the  circuit  is  closed,  the  armature  is  drawn  over  toward  the 
poles,  opening  the  circuit  at  the  spring  contact.  The  coils 
then  immediately  release  the  armature  which  is  drawn  back 
by  means  of  a  coil  spring,  again  closing  the  circuit.  This 
again  energizes  the  coil  and  draws  over  the  magnet,  breaking 
the  circuit,  etc.,  the  cycle  being  completed  with  great  rapidity. 

Mechanical  Generation  of  Electricity 

It  has  been  shown  that  a  current  flowing  through  a  wire 
produces  a  magnetic  field  around  the  wire.  The  converse  of 
this  is  also  true;  that  is,  if  a  wire  is  moved  across  a  magnetic 
field,  Fig.  1':),  or  if  the  wire  remains  stationary  but  a  mag- 
netic field  moves  across  it,  or  any  combination  of  motions 
take  place  such  that  the  wire  cuts  magnetic  lines  of  force,  an 
einf.  will  be  generated  in  the  wire.  Also  if  a  magnetic  field 
which  passes  through  a  coil  of  wire  is  suddenly  created  or 
destroyed,  an  emf.  will  be  generated  in  that  coil. 

The  dynamo  is  an  example  of  a  machine  designed  to  gen- 
erate electricity  by  this  means  of  having  a  wire  cut  through 
a  magnetic  field,  Fig.  24.  Coils  containing  numerous  turns  of 
wire  are  wrapped  around  an  armature  which  rotates  within 
the  magnetic  field  produced  by  other  electromagnets,  in  such 
a  way  that  the  magnetic  lines  of  force  are  cut  by  the  coils 
on  the  armature.  If  the  number  of  lines  of  force  are  in- 
creasing within  a  coil,  the  current  will  be  forced  in  one  di- 
rection. If  the  lines  of  force  are  decreasing  in  the  coil,  the 
current  will  be  forced  in  the  opposite  direction.  The  general 
law  covering  this  is  called  Lenz's  law,  and  is,  that  the  in- 
duced current  is  such  that  it  opposes  the  motion  producing 
it.  Another  statement  of  this  law  is,  that  the  direction  of  any 
induced  current  is  just  the  opposite  of  that  current  which 
would  produce  the  magnetic  lines  of  force  cut  by  the  wire. 

If  the  current  were  lead  off  directly  from  the  coils  of  a 
dynamo  by  means  of  brushes  bearing  on  rings,  to  which  the 
armature  coils  were  connected,  the  current  obtained  would 
flow  first  in  one  direction  and  then  in  the  other,  depending 
on  whether  the  number  of  lines  of  force  cut  was  increasing 
or  decreasing.  In  other  words,  an  alternating  current  would 
be  generated.  To  obtain  direct  current,  a  commutator  is 
placed  on  the  armature  shaft  instead  of  the  rings  and  this 
is  so  made  that  just  as  the  direction  of  current  reverses  in  a 


17 

coil,  the  connections  to  this  coil  are  reversed  so  that  the 
current  obtained  in  the  wires  leading  away  from  the  com- 
mutator is  always  in  the  same  direction,  Fig.  24. 

Generation  of  electricity  by  means  of  the  dynamo  is  one 
form  of  electromagnetic  induction.  Another  form  'of  electro- 
magnetic induction  is  that  made  use  of  in  the  ordinary  in- 
duction coil,  Fig.  25.  In  this  case,  the  coils  remain  stationary, 
but  in  one  coil  the  current  is  turned  on  and  off.  This  alter- 
nately creates  and  destroys  a  magnetic  field.  The  coil  in 
which  the  circuit  is  made  and  broken  is  called  the  "primary" 
coil.  Around  this  coil  is  wound  another  coil  which  is  usually 
made  up  of  a  larger  number  of  turns  and  is  called  the  "sec- 
ondary" coil.  When  the  magnetic  field  is  created  in  the  pri- 
mary coil  due  to  a  flow  of  current,  the  magnetic  lines  of  force 
also  pass  through  the  secondary  coil,  and  induce  an  emf. 
within  it.  When  the  circuit  to  the  primary  coil  is  closed,  the 
magnetic  field  rapidly  builds  up  to  a  certain  maximum  and 
then  remains  constant.  During  the  time  this  magnetic  field 
is  changing  in  magnitude,  an  emf.  is  generated  in  the  sec- 
ondary coil  with  a  magnitude  which  depends  on  the  rate  of 
change  of  the  magnetic  field  due  to  the  primary  coil.  Conse- 
quently when  the  field  reaches  its  maximum  strength,  the  rate 
of  change  is  zero  and  no  further  current  is  produced  in  the  sec- 
ondary coil.  But  when  the  circuit  to  the  primary  is  broken, 
there  is  again  a  change  in  the  number  of  lines  of  force  pass- 
ing through  the  secondary  coil  as  the  magnetic  lines  of  force 
through  the  primary  decrease  from  a  maximum  to  zero  and 
another  impulse  of  current  is  generated  in  the  secondary  dur- 
ing this  change.  The  directions  of  flow  of  the  current  gen- 
erated in  the  secondary  coil  during  the  building  up  of  the 
magnetic  field  and  during  the  falling  off  of  the  magnetic  field 
are  opposite.  The  large  number  of  turns  of  wire  on  the  sec- 
ondary coil,  as  compared  with  the  number  on  the  primary 
coil,  result  in  an  electromotive  force  in  the  secondary  coil 
which  may  be  many  times  as  great  as  that  in  the  primary 
coil.  By  this  ^means  it  is  possible  to  step  up  the  voltage  and 
make  it  high  enough  to  jump  the  high  resistance  of  an  air 
gap  (20,000  volts  are  required  to  jump  a  gap  of  one  inch 
between  needle  points).  If  the  voltage  is  stepped  up  sevoral 
times,  the  current  in  the  secondary  coil  is  correspondingly 
reduced  in  comparison  to  that  in  the  primary,  since  the  power 


IS 


TIC.  25 


]  Core 


Electrostatic 
Charge 


FIG.  26 


Charge  of  a   «• 
Condenser   + 


FIO.  27 


FIG.  30 


Galvanonoter 


FIG  31   ^    Galvnnomoter  Used 
oa  Voltmeter 


Direct  Current 
On  Off 


FIO.  28 


Tiae 


Galvnnonoter  Used 
as  Anaeter 


I — IA/WWAW — , 


FIG.  32 


I © 1 


Anplltude 


Alternating 
Current 


FIG.   29 


Ware  Length 


Spring 


Voltage  Coll 


FIG.  33 


output  of  the  secondary  can  be  no  greater  than  the  input  to 
the  primary,  and  since  the  product  of  the  current  and  voltage 
is  approximately  the  power.  The  high  voltage  obtained  by 
this  means  is  one  of  the  essentials  in  radio  telegraphy  and 
telephony. 

In  order  to  secure  a  practically  continuous  supply  of  the 
high  voltage  current  in  the  secondary  circuit,  it  is  necessary 
to  make  and  break  the  circuit  through  the  primary  coil  very 
rapidly.  To  do  this,  an  armature  and  make  and  break  contact 
system  practically  identical  to  that  used  in  the  ordinary  door 
bell  are  used.  This  alternately  makes  and  breaks  the  current 
through  the  primary  coil  and  creates  and  destroys  the  mag- 
netic field  cutting  the  secondary  coil  and  thereby  induces  the 
high  voltage  current,  flowing  first  in  one  direction  and  then 
in  the  other. 

The  effect  expressed  in  Lenz's  law  is  always  present  when- 
ever there  is  any  change  in  the  strength  of  a  magnetic  field. 
The  force  resulting  from  any  change  is  always  opposite  to 
that  producing  it.  This  means  that  there  is  always  a  certain 
lag  of  the  result  behind  the  change  producing  that  result. 
This  effect  is  greatest  in  coils  wound  over  iron  cores  in  such 
a  way  that  the  lines  of  force  produced  by  all  the  turns  add 
up.  The  property  of  the  coil  which  causes  this  lag  is  called 
the  "reactance"  of  the  coil. 

Characteristics  of  Direct  Currents 

The  whole  science  of  electricity  is  that  of  using  the  various 
phenomena  observed,  in  such  combinations  as  produce  the 
results  desired.  A  direct  current  and  alternating  current  have 
very  different  characteristics.  A  direct  current  requires  a 
complete  electric  circuit  in  order  to  secure  a  flow  in  one 
direction.  This  means  that  none  of  the  electrons  will  start 
to  move  without  disturbing  its  next  door  neighbor,  and  thus 
will  not  move  until  all  can  follow  along  in  one  direction. 
The  electrons  are  impelled  by  some  chemical  or  magnetic 
force  to  start  the  motion.  They  communicate  this  impulse  on 
ahead,  and  if  the  way  is  clear,  that  is,  if  the  circuit  is  com- 
plete, the  motion  of  the  electrons  continues  and  the  direct 
current  flows. 

It  has  been  found  that  electricity  travels  with  a  speed  equal 
to  that  of  light — 300  million  meters  or  186,500  miles  per  sec- 


20 

ond.  This  does  not  mean  that  the  electrons  travels  along  the 
wire  at  that  rate,  but  that  they  communicate  the  impulse  to 
move  at  that  speed.  The  actual  rate  of  motion  of  the  elec- 
trons is  dependent  on  the  magnitude  of  current.  If  a  wire 
186,500  miles  long  were  to  be  had,  an  electron  at  the  far  end 
of  the  wire  would  receive  an  impulse  one  second  after  the 
electron  at  the  first  end  of  the  wire  started  to  move.  This 
speed  seems  incredible,  as  it  really  is,  compared  with  the 
speed  of  which  one  is  accustomed  to  think,  but  it  is  the 
normal  speed  of  electricity.  This  simply  means  that  every- 
thing in  electrical  circuits  takes  place  in  much  shorter  spaces 
of  time  than  one  is  able  to  sense.  But  this  time  element  in- 
volved in  the  flow  of  electric  current,  brief  as  it  is,  is  a  very 
real  factor  and  various  reactions  in  electricity  are  secured, 
particularly  with  respect  to  wireless  work,  only  as  the  cir- 
cuits are  designed  to  take  mto  account  the  time  factor. 

Resistance  in  a  circuit  hinders  the  motion  of  electrons. 
This  property  is  a  means  of  limiting  the  amount  of  motion,  or 
in  other  words,  the  magnitude  of  the  current.  If  there  are 
coils  inserted  in  the  circuit,  the  self-induction  between  the 
turns  will  keep  the  emf.  from  building  up  as  rapidly  as  it 
would  in  a  circuit  without  the  coils.  This  in  some  cases  is 
a  disadvantage,  but  it  frequently  is  a  handy  means  of  pre- 
venting too  sudden  a  flow  of  current  through  one  part  of  a 
system. 

If  a  large  plate,  Fig.  26,  is  connected  at  one  end  of  a  wire, 
and  the  other  end  of  the  wire  connected  to  a  source  of  electro- 
motive force,  there  will  be  a  distribution  of  electrons  all  over 
this  plate.  In  order  to  bring  about  an  increase  in  the  con- 
centration of  electrons  over  the  plate,  the  electrons  coming 
over  the  wire  must  be  made  to  spread  out  over  this  whole 
surface.  This  is  brought  about  by  increasing  the  voltage  im- 
pressed on  the  plate  through  the  wire.  With  a  direct  cur- 
rent connected  to  the  plate  in  this  manner,  there  results  a 
short  impulse  of  electricity  as  the  electrons  spread  out  over 
the  plate  to  charge  it  to  a  potential  corresponding  to  the  volt- 
age applied.  When  this  voltage  is  withdrawn,  this  charge 
flows  back  along  the  line  to  equalize  the  distribution  of  elec- 
trons. If  another  plate  is  placed  opposite  the  first,  Fig.  27, 
an  electric  charge  produced  on  one  of  these  will  induce  an  op- 
posite charge  on  the  other,  due  to  the  fact  that  the  electric 


21 

charge  on  the  first  acts  in  the  same  manner  as  the  magnetic 
pole.  Consequently,  if  the  first  plate  charged  has  a  positive 
potential,  the  'opposite  plate  will  be  charged  negatively  so  that 
the  two  plates  will  attract  each  other.  Such  a  combinaion  is 
ordinarily  called  a  "condenser"  and  the  amount  of  electricity 
represented  by  the  concentration  of  electrons  over  the  sur- 
faces of  the  two  plates  is  called  the  "capacitance"  of  the  con- 
denser. Its  ability  to  hold  electricity  is  a  measure  of  its 
capacitance  and  it  is  expressed  in  units  called  "farads,"  or 
"microfarads"  (mfd.),  .000001  farad.  The  quantity  of  electric- 
ity that  can  be  put  in  a  condenser  may  be  determined  by  the 
equation,  Q  =  E  C,  where  B  is  the  voltage  and  C  is  the  capaci- 
tance. Therefore  the  capacitance  may  be  measured  by 

Q 



— 

E 

Characteristics  of  Alternating  Currents 

Resistance  opposes  the  progress  of  alternating  current  in 
the  same  manner  that  it  does  with  direct  current.  With  di- 
rect current,  any  inductance  in  the  circuit  interferes  with  the 
flow  of  current  only  when  the  circuit  is  made  or  broken  'or 
the  strength  of  current  changed,  while  with  alternating  cur- 
rent this  effect  goes  on  continuously  with  each  change  in  the 
direction  of  the  current.  Consequently,  inductance  has  a 
much  greater  effect  on  the  current  in  the  circuit  and  can 
actually  prevent  an  alternating  current  from  getting  into  a 
circuit  at  all.  This  is  a  very  important  factor,  and  one 
which  is  frequently  very  useful  when  it  is  necessary  to  use 
both  direct  and  alternating  current  in  the  same  circuit. 

An  alternating  current  will  charge  a  condenser  when  the 
current  flows  in  one  direction  and  then  when  the  current  re- 
verses this  charge  will  flow  back  on  the  line,  thereby  helping 
the  current  flow  in  the  other  direction.  The  condenser  then 
also  charges  in  the  opposite  direction  and  discharges  again 
in  the  first  direction.  Therefore,  if  a  condenser  is  of  the 
proper  design,  it  will  offer  no  hindrance  to  the  flow  of  al- 
ternating current  in  a  circuit  including  the  condenser.  Elec- 
trons never  actually  pass  through  the  condenser,  but  their 
effect  does,  so  that  the  "flow"  of  an  alternating  current  may 
be  considered  to  pass  through  a  condenser.  This  difference  in 
the  reaction  of  the  condenser  upon  a  direct  and  upon  an  al- 


22 

ternating  current   is   also   very   useful   in   circuits   in   which 
both   kinds  of  currents  are  employed. 

It  frequently  facilitates  explanation  of  electrical  phenomena 
to  represent  current  and  voltage  values  by  means  of  time 
Curves.  The  voltage  of  a  current  changes  with  the  time. 
Therefore,  if  distances  are  laid  off  on  *a  line  horizontally  to 
represent  time,  then  it  is  possible  to  represent  voltage  values 
during  that  time  by  the  vertical  distances  above  this  hori- 
zontal line  at  the  successive  moments.  For  a  direct  current 
the  voltage  curve  will  show  a  steady  rise  to  its  maximum 
value  and  the  curve  will  then  be  straight  and  parallel  to  the 
time  axis  until  the  current  is  cut  off.  Pig.  28  shows  such  a 
curve  when  there  is  an  inductance  coil  in  the  circuit.  With 
alternating  current,  however,  the  voltage  is  always  changing 
from  the  maximum  in  the  positive  direction  to  the  maximum 
in  the  negative  direction.  This  makes  a  wave-like  curve  when 
plotted  against  time,  Fig.  29.  The  "period"  of  an  alternating 
current  is  the  time  required  for  one  complete  wave  or  cycle; 
that  is,  from  the  point  of  the  horizontal  axis  that  the  curve 
is  increasing  in  the  positive  direction,  until  it  reaches  the 
next  point  on  the  axis  where  it  is  again  rising  in  the  positive 
direction.  Fig.  29  shows  one  complete  wave.  The  "frequency" 
is  the  number  of  these  cycles  which  occur  in  one  second.  The 
"amplitude"  is  the  maximum  value  reached  in  either  the  posi- 
tive or  negative  direction.  The  current  values  can  be  repre- 
sented by  a  similar  curve. 

Measuring  Instruments 

All  the  common  measuring  instruments  have  as  a  basis  the 
galvanometer,  Fig.  30.  This  consists  of  a  small  coil  of  wire 
which  swings  in  a  permanent  magnetic  field.  The  needle  is 
swung  back  to  the  zero  position  by  a  small  hair  spring.  The 
coil  is  suspended  so  that  it  hangs  in  the  zero  position  with  its 
axis  at  right  angles  to  the  magnetic  lines  of  force  produced 
by  the  permanent  magnst.  When  a  small  current  enters  the 
coil,  it  creates  other  lines  of  force  at  right  angles  to  those  of 
the  permanent  magnet,  and  in  consequence  the  coil  tends  to 
turn  to  make  all  lines  of  force  parallel.  As  the  coil  rotates 
about  its  axis,  it  increases  the  tension  on  the  spring.  Hence 
the  amount  of  deflection  is  a  measure  of  the  intensity  of  cur- 
rent flowing  through  the  coil. 


When  the  galvanometer  is  used  as  a  voltmeter,  Pig.  31,  a 
large  resistance  is  placed  in  series  with  it  so  that  at  high 
voltages  only  a  very  small  current  will  flow  through  the  gal- 
vanometer coil.  The  scale  is  graduated  to  read  in  volts.  The 
galvanometer  itself  is  very  sensitive  so  that  care  must  be 
taken  that  only  currents  of  the  order  of  one-thousandth  of  an 
ampere  will  flow  directly  through  the  coil. 

To  use  the  galvanometer  as  an  ammeter  or  current  meas- 
uring instrument,  Pig.  32,  its  terminals  are  placed  across  a 
low  resistance  shunt  which  is  included  in  the  main  circuit. 
The  galvanometer  then  measures  the  I R  drop  (E  =  I R) 
across  the  shunt.  Since  R  is  a  constant  for  the  particular 
shunt  used,  the  galvanometer's  deflection  will  be  proportional 
to  the  current  passing  through  the  resistance. 

To  measure  the  power  being  delivered  by  any  source  of 
electromotive  force,  it  is  necessary  to  measure  both  the  volt- 
age and  amperage.  In  this  case,  instead  of  having  permanent 
magnets  to  create  a  field,  a  stationary  coil  is  connected  across 
the  shunt  which  is  connected  in  series  with  the  circuit.  The 
strength  of  the  magnetic  field  produced  in  this  coil  will  then 
vary  with  the  amount  of  current  flowing  in  the  circuit.  An- 
other coil  is  then  placed  as  with  the  galvanometer  so  that  any 
magnetic  lines  of  force  produced  by  it  will  tend  to  turn  the 
coil  so  that  these  lines  will  be  parallel  with  those  produced 
by  the  current  coil.  This  voltage  coil  is  connected  through  a 
high  resistance  across  the  line.  Consequently  the  resulting 
deflection  is  due  to  the  combined  effect  of  both  the  current 
and  the  voltage  existing  in  the  circuit.  The  instrument  is  so 
designed  that  the  deflection  is  proportional  to  the  product  of 
the  two,  or  in  other  words,  so  that  it  will  read  directly  in 
watts.  Such  an  instrument  is  called  a  "wattmeter,"  Pig.  33. 


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NOV25   1942 


LD  21-10 


Gaylord  Bros. 

Makers 
Stockton,  Calif. 

PAT.  JAN.  2!.  1908 


667306^ 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


